The present invention relates to a method of generating a supercontinuum of optical radiation in an optical waveguide, a supercontinuum optical radiation source, and use thereof, e.g. in telecommunication, spectroscopy, and tomography.
1. The Technical Field
A supercontinuum radiation source is a light source which exhibits broad flat “white” spectrum and laser-like properties of high output power and high degree of spatial coherence compared to thermal light sources.
Conventionally supercontinuum optical radiation is generated by propagating sub-nanoseconds (<10−10 s) single wavelength laser pulses through a nonlinear material. For sufficiently intense (>1010 W) laser pulses, which can be achieved by high power lasers, a supercontinuum of optical radiation will be generated in the nonlinear material. Virtually any nonlinear material can be applied, including e.g. water.
Interactions of the laser radiation and the nonlinear material generating a supercontinuum include nonlinear processes such as self-phase modulation (SPM), cross-phase modulation (XPM), stimulated Raman scattering (SRS), and four-wave mixing (FWM) all of which may contribute to the generated supercontinuum.
For a nonlinear material in form of a high quality optical fibre confining light in a small cross sectional area over long distances, a supercontinuum can be generated by low power laser sources.
For short pulses e.g. less than about 10 picoseconds all of the above-mentioned nonlinear interactions can be responsible for supercontinuum generation in optical fibres. The extent to which each interaction contributes to the spectral broadening is determined by a combination of the characteristics of the laser radiation and the fibre parameters.
In order for four-wave mixing to efficiently contribute to the formation of the supercontinuum the interacting fields must be phase matched. It has long been understood that this requires a pump wavelength close to the zero dispersion wavelength of the optical fibre in use. For standard telecommunications fibre this is usually near 1300 or 1550 nm in the infrared. The recent development of so called photonic crystal, microstructured, or holey fibre has resulted in fibres with much shorter zero dispersion wavelengths around 800 nm. These fibres, combined with expensive mode-locked titanium sapphire pump lasers, have demonstrated broad supercontinuum sources in the visible region of the spectrum.
However, mode locked lasers are complicated and expensive. It is thus desirable to produce supercontinuum sources using longer pulse lasers such as Q-switched lasers or purely continuous wave lasers. For the case of lasers with pulses longer than several tens of picoseconds or continuous wave output, the effects of self and cross-phase modulation on the spectral width of the input pulse are negligible for the purpose of supercontinuum generation. Stimulated Raman scattering and four-wave mixing are thus the interactions which can be utilized.
SRS does not require phase-matching and hence can be used for the generation of new frequencies with basically any pump wavelength/fibre combination. However, the resulting spectrum is not smooth, with peaks at intervals spaced by the characteristic frequency shifts of the fibre material. In addition, the spectrum is primarily shifted to wavelengths longer than the pump.
What is needed is an efficient technique for generating spectrally smooth, single-mode supercontinuum radiation using readily available, low cost, pump lasers.
2. Prior Art Disclosures
U.S. Pat. No. 4,847,850 (Kafka, Linne, and Baer) disclose stimulated Raman scattering using compact acousto-optically Q-switched pump lasers operating at 1064 or 532 nm and standard single-mode fibres. In the case of 532 nm pumping they were able to generate a continuum which extended from the green to the near infrared. With pumping at 1064 nm they demonstrated continua in the infrared which extended to 1500 nm.
Provino et. al. “Compact broadband continuum source based on microchip laser pumped microstructured fiber”, Electronic Lett. 37, 558-559, 2001 discloses visible supercontinuum generation in photonic crystal fibers with a 532 nm passively Q-switched pump laser from JDSU which filters out the longer wavelengths. In addition to SRS they observed four-wave mixing processes phase matched through higher order fibre modes which resulted in the generation of wavelengths shorter than the pump. The supercontinuum extended as short as 400 nm but with low efficiency due to the poor mode overlap of the interacting fields. The output beam was also not diffraction limited which is undesirable for many applications. The characteristic of the fiber used is however not specified.
U.S. Pat. No. 6,097,870 discloses broad band continuum generation in the visible spectrum using an optical fiber with a zero-dispersion at a visible wavelength and use of 100 fs pulses at 780 nm with 8 kW peak power.